The invention relates to an annular permanent magnet field means having circumaxially spaced and inwardly facing pole faces for use in a permanent magnet dynamo-electric machine, to a sub-assembly of magnets used in the fabrication of such a field means, and to a process for making such a field means. More specifically, this invention concerns a dynamoelectric machine field means, and a related sub-assembly and process, of the type having a plurality of magnets arranged in a circular array and fixed to the inner surface of a supporting ring made of ferromagnetic material.
The field means of this invention may be used in either a motor or a generator and may be either a stationary or a rotating part. However, it lends itself particularly well to generators of the type having a rotating field provided by a rotor in the form of a flywheel or other wheel having a rim with magnets on the interior surface thereof, and the invention is, therefore, hereinafter shown and described as applied to such a rotor.
In assembling a permanent magnet field means comprised of a number of magnets in a supporting ring, a manufacturer faces a number of problems. One of these is that of locating the magnets in proper angular relationship with respect to each other. An angular shift of only a few degrees in one of the magnets can markedly decrease the efficiency of the motor or generator in which the field means is used. A second problem involves the fact that the supporting ring is conventionally made of iron and forms part of the flux paths for the magnets. Thus, the magnets should be in close association with the wheel to eliminate high reluctance gaps between the magnets and the wheel and this is sometimes difficult to achieve. A third problem is that manual placement of the magnets in the wheel is expensive. Another problem relates specifically to ceramic magnets, which is the type of magnet usually preferred in the type of field means to which the invention pertains. These magnets are quite brittle, and must be handled with care to avoid cracking, chipping and other damage.
Some of the objects of the present invention are, therefore, to produce an annular permanent magnet field means in which the magnets are accurately located and wherein such accuracy is easily and inexpensively achieved, to produce such a field means in which a high degree of closeness between the magnets and the supporting ring is obtained, and to produce such a field means which may be made with only a small amount of hand labor, as by using a sub-assembly of magnets in the form of a unitary ring which may, if desired, be made on an automatic or semi-automatic assembly machine. A further object is to provide a method of assemblying a field means of the foregoing character whereby the likelihood of accidental magnet damage during assembly is reduced.
In U.S. Pat. No. 3,390,291, a continuous annular magnet ring or sub-assembly consisting of a cluster of magnets and shoes embedded in a continuous carrier of non-magnetic material is held inside a supporting ring which may be part of a flywheel, ring gear or the like, by means of pressure between the magnet ring and the supporting ring. The pressure is obtained by heat shrinking the supporting ring onto the magnet ring. The magnet ring could alternatively be held in place by an adhesive between it and the supporting ring. In either case, however, it is necessary that both the inside diameter of the supporting ring and the outside diameter of the magnet ring be held to close dimensional tolerances to assure a close low reluctance fit between the magnets and the supporting ring. This in turn generally requires that the outer surface of the magnet ring and the inner surface of the supporting ring both be accurately machined prior to assembly of the magnet ring with the supporting ring. In U.S. Pat. No. 3,663,850 and U.S. Pat. No. 3,091,713, various structures are shown for holding arcuate ceramic magnets or the like to the inner surface of a supporting ring. In both of the latter disclosures, resiliently deformable members placed between the ends of the magnets urge the magnets circumferentially apart to cause them to press tightly against the supporting ring. In these devices, however, the magnets and deformable members are somewhat difficult to assemble.